Semiconductor light emitting device having textured structure and method of manufacturing the same

ABSTRACT

A semiconductor light emitting diode having a textured structure and a method of manufacturing the same are provided. The semiconductor light emitting diode includes a first semiconductor layer formed into a textured structure, an intermediate layer formed between the textured structures of the patterned first semiconductor layer, and a second semiconductor layer, an active layer, and a third semiconductor layer sequentially formed on the first semiconductor layer and the intermediate layer.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a divisional of U.S. application Ser. No.11/293,273, filed on Dec. 5, 2005, which claims the benefit of KoreanPatent Application No. 10-2004-0103112, filed on Dec. 8, 2004, in theKorean Intellectual Property Office, the entire disclosure of which areincorporated herein by reference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to a semiconductor light emitting diode,and more particularly, to a semiconductor light emitting diode thatimproves light extraction efficiency using a textured structure and amethod of manufacturing the same.

2. Description of the Related Art

A light emitting diode (LED) is a device used for converting electricalenergy into infra red rays, visible light, or other light using thecharacteristics of a compound semiconductor. The light emitting diode isa type of electro luminescent (EL) device, and presently, the lightemitting diodes that employ an III-V group compound semiconductor arebeing practically utilized.

The III-V group compound semiconductor is a direct transitionsemiconductor, and is widely used for LEDs or laser diodes (LDs) sinceit provides stable operation at a higher temperature than devices thatuse other semiconductors. The III-V group compound semiconductor istypically formed on a substrate formed of sapphire Al₂O₃ or SiC. Toimprove light emission efficiency, or light extraction efficiency, avariety of LED structures have been studied. Presently, a study is beingcarried out to improve the light extraction efficiency by forming atextured structure on a light extraction region of the LED.

Light is hindered at an interface of material layers having differentrefractive indexes according to the refractive index of each of thematerial layers. In the case of a flat interface, when the light passesfrom a semiconductor layer having a greater refractive index (n=2.5)into an air layer having a smaller refractive index (n=1), the lightmust enter the flat interface at less than a predetermined angle withrespect to the normal. If the light enters at an angle greater than thepredetermined angle, the light totally is internally reflected at theflat interface, thereby greatly reducing the light extractionefficiency. To avoid the total internal reflection of light, a method ofincorporating a textured structure at the interface has been attempted.

FIGS. 1A and 1B are cross-sectional views illustrating a conventionallight emitting diode having a textured structure. Referring to FIG. 1A,a p-GaN layer 102, an active layer 103, an n-GaN layer 104 aresequentially formed on a p-electrode 101, and an n-electrode 105 isformed on the n-GaN layer 104. When light generated by the active layer103 is extracted upward through the n-GaN layer 104, to change theincidence angle of the light, a textured structure 106 is incorporatedat an interface between the n-GaN layer 104 and the air layer.

Referring to FIG. 1B, an n-GaN layer 112 is formed on a sapphiresubstrate 111, and an n-AlGaN layer 113, an active layer 114, a p-AlGaNlayer 115, a p-GaN layer 116, and a p-electrode 117 are sequentiallyformed on a region of the n-GaN layer 112. An n-electrode 118 is formedon a region of the n-GaN layer 112 where the n-AlGaN layer 113 is notformed. This is a flip-chip structure in which light generated by theactive layer 114 is mainly extracted through the transparent sapphiresubstrate 111. Here, the light extraction efficiency is improved byforming a textured structure 120 on the surface of the sapphiresubstrate 111.

A conventional semiconductor light emitting diode incorporates thetextured structure 120 to improve the light extraction efficiency.However, particularly as depicted in FIG. 1B, when the texturedstructure 120 is incorporated by patterning the sapphire substrate 111,the growth of a semiconductor layer with a uniform quality is difficult,because there is a great possibility of generating defects in thesemiconductor layer due to unmatched crystal structure between thesapphire substrate 111 and the semiconductor layer formed on thesapphire substrate 111. As a result, the light extraction efficiency isreduced due to the internal crystal defects.

SUMMARY OF THE DISCLOSURE

The present invention may provide a semiconductor light emitting diodehaving a structure that can improve light extraction efficiency andreduce internal crystal defects of the semiconductor light emittingdiode and a method of manufacturing the semiconductor light emittingdiode.

According to an aspect of the present invention, there may be provided asemiconductor light emitting diode comprising: a first semiconductorlayer formed to a textured structure; an intermediate layer formedbetween the textured structures of the patterned first semiconductorlayer; and a second semiconductor layer, an active layer, and a thirdsemiconductor layer sequentially formed on the first semiconductor layerand the intermediate layer.

The substrate may be a sapphire substrate.

The intermediate layer may be formed of a transparent insulatingmaterial or a transparent conductive material having a refractive indexof 2.5 or less.

The intermediate layer may be formed of a transparent insulatingmaterial including at least one of SiO₂, SiN_(x), Al₂O₃, HfO, TiO₂, orZrO.

The intermediate layer may be formed of a transparent conductivematerial, such as ZnO or an In oxide that includes at least one additiveselected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se,Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La.

The first semiconductor layer, the second semiconductor layer, and thethird semiconductor layer may be formed of GaN.

The first semiconductor layer and the intermediate layer may be formedon the sapphire substrate.

The semiconductor light emitting diode may further comprise a firstelectrode formed on the third semiconductor layer, and a secondelectrode formed on a region of the second semiconductor layer in whichthe active layer is not formed.

The width of the textured structure of the first semiconductor layerpattern may be gradually narrowed as it goes upward.

According to another aspect of the present invention, there is provideda method of manufacturing a semiconductor light emitting diode having atextured structure, the method comprising: forming a first semiconductorlayer on a sapphire substrate; exposing a portion of the sapphiresubstrate while forming the textured structure by etching the firstsemiconductor layer; forming an intermediate layer on the exposedsapphire substrate between the textured structures of the firstsemiconductor layer; and sequentially forming a second semiconductorlayer, an active layer, and a third semiconductor layer on the firstsemiconductor layer and the intermediate layer.

The exposing of a portion of the sapphire substrate while forming thetextured structure by etching the first semiconductor layer maycomprise: performing a first etching to form etch pits on the surface ofthe first semiconductor layer; and performing a second etching to exposethe surface of the sapphire substrate by etching the etch pits of thefirst semiconductor layer.

The first etching may be performed using H₃PO₄ and the second etchingmay be performed using KOH.

The forming of an intermediate layer on the exposed sapphire substratebetween the textured structures of the first semiconductor layercomprises: coating a optical transmittance material on the exposedsapphire substrate and the textured structure of the first semiconductorlayer; and forming the intermediate layer by leveling the opticaltransmittance material to expose the surface of the first semiconductorlayer.

The method may further comprise annealing after the opticaltransmittance material is coated on the exposed sapphire substrate andthe textured structure of the first semiconductor layer.

The method may further comprise performing a third dry etching of theexposed surface of the sapphire substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill be described in greater detail in exemplary embodiments thereofwith reference to the attached drawings in which:

FIGS. 1A and 1B are cross-sectional views of conventional semiconductorlight emitting diodes having a textured structure;

FIGS. 2 and 3 are cross-sectional views of semiconductor light emittingdiodes having a textured structure according to the present invention;

FIGS. 4A through 4E are cross-sectional views for illustrating a methodof manufacturing a semiconductor light emitting diode according to anembodiment of the present invention;

FIGS. 5A through 5E are cross-sectional views for illustrating a methodof manufacturing a semiconductor light emitting diode according toanother embodiment of the present invention;

FIGS. 6A through 6D are SEM images of the semiconductor light emittingdiode according to an embodiment of the present invention; and

FIG. 7 is a graph showing the light extraction efficiencies of aconventional semiconductor light emitting diode having a texturedstructure and a semiconductor light emitting diode having a texturedstructure according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention will now be described more fully with reference tothe accompanying drawings in which exemplary embodiments of theinvention are shown.

FIGS. 2 and 3 are cross-sectional views of semiconductor light emittingdiodes having a textured structure according to the present invention.

FIG. 2 shows a textured structure applied to flip-chip typesemiconductor light emitting diodes and FIG. 3 shows a texturedstructure applied to vertical type semiconductor light emitting diodes.

Referring to FIG. 2, a first semiconductor layer 22 and an intermediatelayer 23 are formed in a textured structure on a transparent substrate21, and a second semiconductor layer 24 is formed on the firstsemiconductor layer 22 and the intermediate layer 23. An active layer25, a third semiconductor layer 26 and a first electrode 27 aresequentially formed on a first region of the second semiconductor layer24. A second electrode 28 is formed on a second region of the secondsemiconductor layer 24.

The materials used for forming the layers are described hereafter. Thetransparent substrate 21 can be a widely used sapphire Al₂O₃ substrate,and the first semiconductor layer 22 and the second semiconductor layer24 can be formed of p-GaN. The intermediate layer 23 may be formed of atransparent insulating material or a transparent conductive materialhaving a refractive index of 2.5 or less. For example, the transparentinsulating material can be SiO₂, SiN_(x), Al₂O₃, HfO, TiO₂, or ZrO, andthe transparent conductive material can be ZnO or an In oxide thatincludes at least one additive selected from the group consisting of Mg,Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg,Pr, and La. Here, it is seen that the intermediate layer 23 is formed ofa transparent material. The active layer 25 can be formed of a materialtypically used for forming a semiconductor light emitting diode or alaser emitting diode in a multi-layer structure of a multi-quantum wellbarrier structure. The third semiconductor layer 26 can be formed ofp-GaN, and, at this time, the first electrode 27 is formed of a p-typeconductive material and the second electrode 28 is formed of an n-typeconductive material.

As shown in FIG. 2, in the textured structure according to an embodimentof the present invention, the intermediate layer 23 is formed within aregion where the first semiconductor layer 22 is patterned into thetextured structure. Here, the distance between the pattered texturedstructures of the first semiconductor layer 22 is not uniform, but isdetermined according to crystal defects in the first semiconductor layer22, particularly to screw dislocation which will be described later withreference to a subsequent manufacturing process. According to thesemiconductor light emitting diode having the above structure, theintermediate layer 23 is formed in the crystal defect region of thefirst semiconductor layer 22, and the internal crystal defects can bereduced by forming the second semiconductor layer 24 on the firstsemiconductor layer 22. Accordingly, the extraction efficiency of lightgenerated by the active layer 25 can be increased by incorporating thetextured structure.

FIG. 3 is a cross-sectional view of a vertical type semiconductor lightemitting diode having a textured structure according to an embodiment ofthe present invention. Referring to FIG. 3, a first electrode 32, athird semiconductor layer 33, an active layer 34, and a secondsemiconductor layer 35 are sequentially formed on a lower structure 31.A first semiconductor layer 37 patterned into a textured structure layerand an intermediate layer 36 are formed on the second semiconductorlayer 35. Also, a second electrode 38 is formed on the firstsemiconductor layer and the intermediate layer 36.

The materials for forming each of the layers that constitute thevertical type semiconductor light emitting diode are as follows. Thefirst semiconductor layer 37 and the second semiconductor layer 35 canbe formed of p-GaN. The intermediate layer 36 may be formed of atransparent insulating material or a transparent conductive materialhaving a refractive index of 2.5 or less. For example, the transparentinsulating material can be SiO₂, SiN_(x), Al₂O₃, HfO, TiO₂, or ZrO, andthe transparent conductive material can be ZnO or an In oxide thatincludes at least one additive selected from the group consisting of Mg,Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg,Pr, and La. The active layer 34 can be formed of a material typicallyused for forming a semiconductor light emitting diode or a laseremitting diode in a multi-layer structure of a multi-quantum wellbarrier structure. The third semiconductor layer 33 can be formed ofp-GaN, and, at this time, the first electrode 32 is formed of a p-typeconductive material and the second electrode 38 is formed of an n-typeconductive material.

As shown in FIG. 3, in the textured structure according to an embodimentof the present invention, the intermediate layer 36 is formed within aregion where the first semiconductor layer 37 is patterned into thetextured structure. Here, the distance between the patterned texturedstructures of the first semiconductor layer 37 is not uniform, but isdetermined according to crystal defects in the first semiconductor layer37, particularly, to screw dislocation. According to the semiconductorlight emitting diode having the above structure, the intermediate layer36 is formed in the crystal defect region of the first semiconductorlayer 37, and the internal crystal defects can be reduced by forming thesecond semiconductor layer 35 on the first semiconductor layer 37.Accordingly, the extraction efficiency of light generated by the activelayer 33 can be increased by incorporating the textured structure.

A method of manufacturing a semiconductor light emitting diode having atextured structure according to an embodiment of the present inventionwill now be described with reference to FIGS. 4A through 4E.

Referring to FIG. 4A, a first semiconductor layer 42 is formed on asubstrate 41. Here, the substrate 41 is a sapphire substrate having arefractive index of 1.78, and the first semiconductor layer 42 is formedof n-GaN. After the first semiconductor layer 42 is applied, a firstetching process for etching the surface of the first semiconductor layer42 is performed using H₃PO₄. Here, internal crystal defects may begenerated since the sapphire has a different crystal structure than GaN.Particularly, internal crystal defects which grow vertically from thesapphire substrate 41 toward the first semiconductor layer 42, such asscrew dislocation 43, can be formed. When the surface of the firstsemiconductor layer 42 is wet etched using H₃PO₄, etch pits are formedat screw dislocation 43 regions since the etching occurs mainly at thescrew. The wet etching progresses downward in the direction of the screwdislocations 43 and in the lateral directions as well. FIG. 6A is a SEMimage of the first semiconductor layer 42 after the wet etching processas depicted in FIG. 4A with respect to the first semiconductor layer 42is performed using H₃PO₄.

Referring to FIG. 4B, a second etching is performed with respect to thefirst semiconductor layer 42 using KOH. When the second etching isperformed using KOH, the etching progresses vertically downward alongthe screw dislocations 43 of the first semiconductor layer 42. Theetching direction of the first semiconductor layer 42 by KOH isvertically downward unlike that obtained by the use of H₃PO₄.Consequently, the surface of the sapphire substrate 41 is exposed, andthe cross-section of the first semiconductor layer 42 becomes a texturedstructure patterned into a trapezoidal shape. Reference numeral 42 arepresents a region etched by H₃PO₄ and KOH. FIG. 6B is a SEM image forshowing the result of etching the first semiconductor layer 42.Referring to FIG. 6B, the cross-section of the first semiconductor layer42 has a trapezoidal shape, that is a textured structure, as a result ofetching the first semiconductor layer 42 using KOH until the surface ofthe sapphire substrate 41 is exposed.

Referring to FIG. 4C, an intermediate layer 44 is formed on the firstsemiconductor layer 42 patterned into a textured structure on thesapphire substrate 41. The intermediate layer 44 may be formed of amaterial having a high light transmittance since light emitted by anactive layer is extracted to the outside through the textured structure.The intermediate layer 44 may be formed of a transparent insulatingmaterial or a transparent conductive material having a refractive indexof 2.5 or less. For example, the transparent insulating material can beSiO₂, SiN_(x), Al₂O₃, HfO, TiO₂, or ZrO, and the transparent conductivematerial can be ZnO or an In oxide that includes at least one additiveselected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se,Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. These materialshave refractive indexes in the range of approximately 1.4 to 1.8. FIG.6C is a SEM image of the intermediate layer 44 formed on the firstsemiconductor layer 42 patterned into the textured structure. Theintermediate layer 44 is formed in the etched region of the firstsemiconductor layer 42 as well as on the first semiconductor layer 42.After the intermediate layer 44 is applied, an annealing process canfurther be performed. A MOCVD process an be performed at 1100° C. forapproximately 1 hour under a H₂ atmosphere.

Referring to FIG. 4D, to expose the upper part of the firstsemiconductor layer 42 patterned into the textured structure, a levelingprocess is performed to remove the upper part of the intermediate layer44. Accordingly, the intermediate layer 44 remains only between thetextured structures of the first semiconductor layer 42.

Referring to FIG. 4E, a second semiconductor layer 45 is formed on theexposed first semiconductor layer 42 and the remaining intermediatelayer 44. The second semiconductor layer 45 may be formed of the samematerial as the first semiconductor layer 42, such as n-GaN. In thiscase, since the second semiconductor layer 45 is grown on the firstsemiconductor layer 42 which has fewer crystal defects than the sapphiresubstrate 41, the crystal defects in the second semiconductor layer 45are greatly reduced compared to the instance when the secondsemiconductor layer 45 is grown directly on the sapphire substrate 41.

The textured structure according to an embodiment of the presentinvention can be formed in a semiconductor light emitting diode by theprocesses described with reference to FIGS. 4A through 4E. An activelayer and a third semiconductor layer formed on the second semiconductorlayer 45 can be readily formed using a conventional process. Thetextured structure can be used as a flip-chip structure, or as avertical structure after the sapphire substrate 41 is removed andelectrodes are further formed.

The textured structure formed in the semiconductor light emitting diodeusing the processes described above, unlike in the conventional art, notonly improves the light extraction efficiency but also reduces crystaldefects, thereby allowing stable operation and extending the lifetime ofthe device.

A semiconductor light emitting diode having a textured structureaccording to another embodiment of the present invention will now bedescribed with reference to FIGS. 5A through 5E.

Referring to FIG. 5A, a first semiconductor layer 52 is formed on asubstrate 51. Here, the substrate 51 is a sapphire substrate, and thefirst semiconductor layer 52 is formed of n-GaN. After the firstsemiconductor layer 52 is applied, a first etching process is performedusing H₃PO₄ to etch the surface of the first semiconductor layer 52.This forms etch pits at screw dislocation 43 regions, since the etchingoccurs mainly at the screw dislocation 43 regions. The wet etchingprogresses downward in the direction of the screw dislocations 43, andin the lateral directions as well.

Referring to FIG. 5B, a second etching is performed on the firstsemiconductor layer 52 using KOH. This etches vertically downward alongthe screw dislocations 43 of the first semiconductor layer 52. Theetching direction of the first semiconductor layer 52 by KOH isvertically downward, unlike that obtained by the use of H₃PO₄.Consequently, the surface of the sapphire substrate 51 is exposed, andthe cross-section of the first semiconductor layer 52 becomes a texturedstructure patterned into a trapezoidal shape. Reference numeral 52 arepresents a region etched by H₃PO₄ and KOH. At this time, after etchingthe first semiconductor layer 52 by KOH, etching is performed on aregion of the substrate 51 exposed by the dry etching. Accordingly,grooves are formed by etching the exposed regions of the substrate 51.FIG. 6D is a SEM image of the substrate 51 obtained dry etching afterthe etching of the first semiconductor layer 52 using KOH.

Referring to FIG. 5C, an intermediate layer 54 is formed on the firstsemiconductor layer 52 patterned into a textured structure on thesapphire substrate 51. The intermediate layer 54 may be formed of atransparent insulating material or a transparent conductive materialhaving high light transmittance and a refractive index of 2.5 or less,since light emitted by an active layer is extracted to the outsidethrough the textured structure. The transparent insulating material canbe SiO₂, SiN_(x), Al₂O₃, HfO, TiO₂, or ZrO, and the transparentconductive material can be ZnO or an In oxide that includes at least oneadditive selected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr,Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. After theintermediate layer 54 is coated, an annealing process can further beperformed. The annealing can be performed at approximately 1100° C. forapproximately 1 hour under a H₂ atmosphere.

Next, referring to FIG. 5D, to expose the upper part of the firstsemiconductor layer 52 patterned into the textured structure, a levelingprocess is performed to remove the upper part of the intermediate layer54. Accordingly, the intermediate layer 54 remains only between thetextured structures of the first semiconductor layer 52.

Referring to FIG. 5E, a second semiconductor layer 56 is formed on theexposed first semiconductor layer 52 and the remaining intermediatelayer 54. The second semiconductor layer 56 may be formed of the samematerial as the first semiconductor layer 52, such as n-GaN. In thiscase, since the second semiconductor layer 56 is grown on the firstsemiconductor layer 52 which has fewer crystal defects than the sapphiresubstrate 51, the crystal defects in the second semiconductor layer 56are greatly reduced compared to the instance when the secondsemiconductor layer 56 is grown directly on the sapphire substrate 51.An active layer and a third semiconductor layer formed on the secondsemiconductor layer 56 can be readily formed using a conventionalprocess.

FIG. 7 is a graph showing the light extraction efficiencies of aconventional semiconductor light emitting diode having a texturedstructure and a semiconductor light emitting diode having a texturedstructure according to an embodiment of the present invention.

Referring to FIGS. 2 and 3, the textured structure of the firstsemiconductor layer 22 has a hexagonal trapezoidal shape or a hexagonalcylindrical shape, or an inverse of these shapes. Patterns are prepared,each with a diameter of 1 μm, and a height of 0.5 μm, and with thedistance between the patterns being 1 μm, and the light extractionefficiency of the patterns is investigated. The results show that thesemiconductor light emitting diode having the textured structure(dielectric embedded nitride structure, n=1.4) according to anembodiment of the present embodiment has a maximum of 85% higher lightextraction efficiency than a conventional planar structure semiconductorlight emitting diode, and a maximum of 77% higher light extractionefficiency than a semiconductor light emitting diode Ref 2 having aconventional trapezoidal textured structure (patterned sapphiresubstrate (PSS), n=1.78).

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

According to the present invention, the extraction efficiency of lightemitted by an active layer can be greatly improved, and the crystaldefects in the semiconductor device can be reduced, by forming atextured structure pattern in a semiconductor layer of a semiconductorlight emitting diode, thereby enabling stable operation and increasingthe lifespan of the semiconductor light emitting diode.

1. A method of manufacturing a semiconductor light emitting diode havinga textured structure, the method comprising: forming a firstsemiconductor layer on a sapphire substrate; exposing a portion of thesapphire substrate while forming the textured structure by etching thefirst semiconductor layer; forming an intermediate layer on the exposedsapphire substrate between the textured structures of the firstsemiconductor layer; and sequentially forming a second semiconductorlayer, an active layer, and a third semiconductor layer on the firstsemiconductor layer and the intermediate layer.
 2. The method of claim1, wherein the exposing of a portion of the sapphire substrate whileforming the textured structure by etching the first semiconductor layercomprises: performing a first etching to form etch pits on the surfaceof the first semiconductor layer; and performing a second etching toexpose the surface of the sapphire substrate by etching the etch pits ofthe first semiconductor layer.
 3. The method of claim 2, wherein thefirst etching is performed using H₃PO₄ and the second etching isperformed using KOH.
 4. The method of claim 1, wherein the forming of anintermediate layer on the exposed sapphire substrate between thetextured structures of the first semiconductor layer comprises: coatinga transparent material on the exposed sapphire substrate between thetextured structures of the first semiconductor layer; and forming theintermediate layer by leveling the transparent material to expose thesurface of the first semiconductor layer.
 5. The method of claim 4,wherein the intermediate layer is formed of a transparent insulatingmaterial including at least one of SiO₂, SiN_(x), Al₂O₃, HfO, TiO₂, orZrO.
 6. The method of claim 1, wherein the intermediate layer is formedof a transparent conductive material, such as ZnO or an In oxide thatincludes at least one additive selected from the group consisting of Mg,Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg,Pr, and La.
 7. The method of claim 4, further comprising annealing afterthe transparent material is coated on the exposed sapphire substratebetween the textured structures of the first semiconductor layer.
 8. Themethod of claim 2, further comprising performing a third dry etching theexposed surface of the sapphire substrate.